Augmented Reality (AR) is a technology that is closely related to Virtual Reality (VR). The user does not immerse completely into a virtual, computer generated environment, instead he stays in his real environment that gets augmented with virtual objects. These virtual objects are superimposed as accurately as possible into the user's field of view. Important classic fields of research in the area of AR are the so-called tracking, i.e. the determination of the user's field of view, the correct illlumination of virtual objects, as well as shadow casting and occlusion between virtual and real objects. In recent years, another question attracted more and more attention: How to design a versatile software architecture for AR systems. This dissertation focuses on that question. Initially, system architectures of existing VR and AR systems are analyzed. Then, starting from the existing VR standard VRML resp. its successor X3D, it is studied how to extend this standard to fulfill the special needs of mobile AR applications, focussing on the device management, on the communication in the network, and on the development and debugging of mobile AR applications.